1. Field of the Invention
The present invention relates to footwear, specifically an improved sandal with a flexibly-hinged, three-part, extended, rigid sole system that is designed to enable normal, full-stride walking on very soft sand.
The present invention additionally relates to footwear, specifically a sandal designed for use with feet that are also clad in conventional walking shoes or jogging sneakers. The invention's objective of eliminating deep foot sinking is achieved by employing a flexibly-hinged, three-part, rigid sole system comprised of three, rigid sole sections: a forward sole section, a middle sole section, and a rear sole section.
2. Description of the Prior Art
Rigid sole sandals and clogs have long been the footwear of choice for walking on very soft sand, like that typically found at beach and desert locations. The reason for this preference is that conventional, rigid sole sandals and clogs, as opposed to conventional, flexible sole sandals, function significantly better in preventing foot sinking. But, this superior performance occurs only when the sandal-wearer is engaged in short-stride walking, not full-stride walking. Prior to the present invention there has never been a sandal, flexible sole or rigid sole, that could eliminate, or even significantly reduce, deep foot sinking during full-stride walking without also being accompanied by very serious, negative side effects such as having to walk with one's feet positioned further apart than normal. During past history some inventors must surely have attempted to solve the foot-sinking problem in the most obvious way, by designing sandals with over-sized soles, not too unlike old-fashioned snow shoes that look like tennis rackets, that indeed would eliminate deep foot sinking in soft sand during full-stride walking. Most such solutions, however, would also have been accompanied by very serious side effects such as awkwardness and discomfort when walking, which would explain why we do not see any such products on the market today. Beach goers who prefer full-stride walking to short-stride walking, have traditionally had to restrict their walking to the water's edge where the sand is firmer and where there is little foot sinking.
The conventional, rigid sole sandal or clog that is most commonly used for walking on very soft sand is comprised of a one-part, rigid sole and a thong for securing that sole to a person's foot. The size of the sole universally conforms, more or less, to the size and shape of the sandal-wearer's foot; and the foot's heel is always positioned very close to the back edge of the sole.
The foot-sinking problem can be better understood by analyzing the performance of a conventional, one-part, rigid sole sandal or clog during the various stages of a full-stride walking cycle, as explained below.
When wearing conventional, one-part, rigid sole sandals or clogs and engaged in full-stride walking, vs. short-stride walking, the sandal-wearer's upper body begins the cycle by leaning forward as a foot is thrust forward in the air, above the soft sand, toward a first step. At the same moment, the trailing foot responds to the body's forward motion by instinctively arcing its heel and instep upward, away from the back portion of the sandal sole as half of the sandal-wearer's body's weight, which had been supported by the heel, shifts forward to the ball joints of the toes, and with a small amount to the toes themselves.
As the sandal-wearer's body and outstretched leading leg continue their forward movement, the heel and instep of the trailing foot continue their arcing motion upward as the instep begins pressing firmly against the thong straps that wrap across and around it. But, because the body's full weight is pressing heavily down upon the forward portion of the sandal's one-part, rigid sole, the entire sole is held down flatly and firmly upon the soft sand, causing the tension upon the thong straps to become greater and greater. After causing considerable foot discomfort the ever-increasing tautness of the thong straps eventually overcomes the pressure of the sandal-wearer's heavy body weight pressing the forward portion of the one-part, rigid sole firmly upon the sand.
As the back portion of the sandal sole is forced up above the soft sand by the pulling thong straps, at an angle of roughly 15 to 25 degrees, the entire sole (which is constructed of one rigid piece) is forced up at that angle. When the tilting occurs, the front end of the sole immediately punches deeply down into the soft sand (approximately 1.25 to 2 inches, or more) as a result of: (1) the heavy, body weight pressure that continues to be concentrated at the forward end of the sandal sole; and (2) the 15- to 25-degree angle of the sandal sole.
When the rear portion of the sandal sole is forced up above the soft sand at an angle, it leaves the forward portion of the sandal with an insufficient number of square inches of sole surface area in contact with the soft sand to fully support the body's weight. Consequently, the forward end of the sole immediately punches down into the soft sand until enough square inches of sole surface area are once again in contact with the soft sand to support the sandal-wearer's heavy body weight.
It should be noted that when any flat plane comes in contact with soft sand while positioned at an angle, its weight-supporting capability per-square-inch of surface contact is diminished; and when the angle of the plane increases, its weight-supporting capability decreases—an inverse proportion. To be precise, the 15- to 25-degree angle of the sole reduces the sole's weight-supporting capability by 16.7% to 27.8%, respectively. Immediately after that abrupt foot sinking of the trailing foot, the sandal-wearer's full-body weight is instinctively shifted from the trailing foot, forward to the heel of the leading foot as it strikes the soft sand ahead at a 15- to 25-degree angle at the completion of its airborne stepping action forward. And, for basically the same reasons listed for the trailing foot above, the leading foot sinks down deeply (approximately 1.25 to 2 inches, or more) into the soft sand ahead because: (1) the heavy, body weight pressure is concentrated totally upon the foot's heel which is positioned at, or very near, the back edge of the sandal sole; and (2) the sandal sole is positioned at a 15- to 25-degree angle, in its relationship to the surface of the sand.
When the rear portion of the sandal sole strikes the soft sand at an angle, it has an insufficient number of square inches of sole surface area in contact with the soft sand to fully support the body's weight because the forward portion of the sandal is still positioned above the soft sand. Consequently, the back end portion of the sole punches down deeply into the soft sand until enough square inches of sole surface area come in contact with the soft sand to support the sandal-wearer's heavy body weight. Had the stepping action been a short-stride step, vs. a full-stride step, the leading leg and foot, as well as the trailing leg and foot, would have been positioned at a much lesser angle, and consequently the sandal-wearer would have experienced very little foot sinking, and very little, or no, foot discomfort due to thong pressures.
The degree of foot sinking that occurs on very soft sand is directly proportional to the angle of the sandal sole, and indirectly proportional to the square-inch area of sandal sole that is in contact with the soft sand. In further explanation, it should be noted that during full-stride walking, when the leading leg is thrust forward at a 15- to 25-degree angle, in its relationship to the vertical, the sandal-wearer's foot and sandal sole both strike the soft sand at approximately that same 15- to 25-degree angle, but in relationship to the soft sand, not the vertical, maintaining the anatomically natural and instinctive 90-degree angle relationship between the leg and the foot. In further explanation still, when a leg is thrust forward in the air toward a new step, a person's body instinctively locks the foot of that leg at an angle of approximately 90 degrees, in its relationship to the leg, in anticipation of the person's heavy body weight being totally concentrated upon the heel of the foot upon impact with the soft sand ahead.
Wearers of conventional, rigid sole sandals avoid the foot-sinking problem by simply engaging in short-stride steps only. This action results in the outstretched legs and feet being positioned at a much lesser angles, and consequently very little foot sinking.
The foot-sinking problem can also be better understood by analyzing the performance of a conventional, walking shoe during the various stages of a full-stride walking cycle.
When wearing conventional walking shoes or jogging sneakers, while engaged in full-stride walking or jogging on very soft sand, as the sandal-wearer's upper body moves forward to take a step, a foot is thrust forward in the air above the sand's surface. But, before that leading foot makes contact with the soft sand ahead, the heel of the trailing foot instinctively arcs upward, away from the surface of the soft sand, in preparation for that foot being lifted airborne off the soft sand and thrust forward toward its own next step.
As the foot's heel arcs upward, the sandal-wearer's total body weight becomes concentrated totally upon the ball joints of the toes, and with a small amount concentrated upon the toes themselves. As the sandal-wearer's upper body continues its move forward, and the leading leg is nearing completion of its airborne stepping action forward, the heel of the trailing foot arcs upward even higher, and the angle of each of the sandal-wearer's outstretched legs, in their relationship to the vertical, becomes quite pronounced—roughly 15 to 25 degrees. But, before the heel of the trailing foot completes its arcing movement upward, the sole of that shoe reaches its maximum degree of flexibility, thereby causing the front portion of the shoe sole to cease flexing and begin tilting downward into the soft sand as the heel continues its arcing movement upward.
By the time-the-heel completes its upward arc, the front portion of the foot's shoe sole has tilted downward in front at an angle of roughly 15 to 25 degrees, and there is an insufficient number of square-inches of sole surface area in contact with the soft sand to support the body's full weight. As a result, 1.25 inches to 2.5 inches, more or less, of foot sinking results before there are enough square-inches of sole surface area in contact with the soft sand to support the body's weight.
Had the trailing foot been positioned at a much lesser angle, as it is during short-stride walking, very little less foot sinking would have occurred. The amount of foot sinking that occurs on very soft sand is directly proportional to the angle of the shoe sole, in its relationship to the surface of the soft sand, and is indirectly proportional to the square-inch surface area of that portion of the shoe sole that remains in contact with the soft sand.
Wearers of the conventional, flexible-sole shoes being discussed, are forced to walk with short strides, vs. full strides, to cause the shoe sole to be positioned at a much lesser angle, and consequently cause much less foot sinking to occur before enough square-inches of sole surface area are in contact with the soft sand to support the body's weight. In this particular instance, immediately following the foot sinking that occurs with the trailing foot, the beach walker's full-body weight is instinctively shifted away from the ball joints of the toes portion of the trailing foot, forward to the heel of the leading foot, as it strikes the soft sand ahead while being positioned at a 15 to 25 degree angle, at the completion of its airborne stepping action forward, as explained more fully in the next paragraph.
When the leading foot strikes the soft sand, its angular orientation to the surface of the soft sand, causes it to sink down into the soft sand until enough square-inches of the sole's surface area come in contact with the soft sand to support the sandal-wearer's full-body weight. The amount of foot sinking that occurs depends upon the severity of the foot angle and the body weight of the individual. The less the angle, the less the foot sinking, and the less the body weight, the less the foot sinking; but the sinking normally ranges from roughly 1.25 inches to 2.5 inches.
The phenomena of the sandal-wearer's foot changing its angle of orientation, in its relationship to the sand's surface, occurs during short-stride walking cycles also, but to a much lesser extent. In explanation, when a leg is thrust forward in the air toward a new step forward, the human body instinctively locks the foot of that leg at its most natural anatomical angle (approximately 90 degrees), in its relationship to the leg, in anticipation of the body's full weight being concentrated upon the heel of the sandal-wearer's foot when the foot and sandal contact the soft sand ahead. And, during full-stride walking, because the leg is thrust forward at a 15 to 25 degree angle, in its relationship to vertical, the sandal-wearer's foot and sandal sole strike the soft sand at approximately that same 15 to 25 degree angle, in its relationship to the soft sand.
During short-stride walking the leg is thrust forward at a much lesser angle; and, consequently, the sandal-wearer's foot and sandal sole strike the soft sand at roughly that same lesser angle, thereby causing a much smaller amount of foot sinking to occur. It should be noted that the support capability of a square-inch of a shoe's sole on very soft sand is diminished by almost the same percentage as the angle of the sole, in its relationship to the sand's surface. A 15- to 25-degree angle reduces the support capability of the footwear sole by 16.7 to 27.8 percent. A 45-degree angle reducing the support capability of a sole by 50 percent. A hypothetical 90-degree angle reduces the support capability of a sole by 100 percent.
Also, when a shoe's sole makes contact with very soft sand at the completion of the airborne stage of a full-stride, stepping action forward, its 15- to 25-degree angle orientation with the sand, causes only a miniscule amount of surface contact to take place initially along the back edge of the shoe sole. The small area of surface contact results in almost no initial sand resistance. But, as the body's heavy weight forces the sandal's sole to sink down deeply into the soft sand, more and more of the shoe's sole surface area comes in contact with the soft sand, until there are enough square-inches of sole contact with the soft sand to support the body's full weight, and the sinking stops.
For example, when contact is made with the foot positioned at an 18-degree angle, the shoe's sole must sink down approximately 1.625 inches before 5 linear inches of the shoe's sole come into contact with the soft sand. When contact is made with the foot positioned at a 25-degree angle, the sandal sole must sink down approximately 2.3 inches before 5 linear inches of sole come in contact with the soft sand. The actual number of linear inches needed to support a sandal-wearer's full weight, of course, depends upon the shoe's width, the sandal's angular orientation, and the body weight of the sandal-wearer.
Although hinged, multi-section, rigid sole footwear dates back to several patents of the early 1900s (and possibly before), the idea has never been employed for any purpose other than to alleviate the foot discomfort caused by the inflexibility of rigid-sole sandals and clogs. In those early patents, the hinged, two-part (or more) rigid sole systems introduced flexibility to rigid sole footwear, making the walking process much less cumbersome and much more comfortable by making the movements of the sole conform more to the foot's movement, thereby eliminating most of the thong pressures created by one-piece, rigid sole footwear.
In the present invention, however, the flexibly-hinged, three-part, rigid sole system not only eliminates foot discomfort, but also works in tandem with the novel idea of added extra sole length behind the sandal-wearer's heel to make possible the employment of inverted mechanical leverage to greatly reduce deep foot sinking.
The present invention, with its extended sole length and extra sole width, looks like an over-sized sandal and is quite unconventional and strange-looking, but it functions exceptionally well, and allows beach- and desert-lovers to walk normally on very soft sand with almost the same degree of ease and comfort as walking on firmer surfaces. With the present invention there is no need to be confined to short-stride steps to avoid foot sinking, and there is no need to concentrate on one's walking in an effort to lessen foot sinking.
Thus, a sand walking sandal solving the aforementioned problems is desired.